Leaf Pathogen Research Articles

Patterns of leaf‐pathogen infection in the understory of a Mexican rain forest: incidence, spatiotemporal variation, and mechanisms of infection

This study assessed the levels of damage by leaf pathogens and their variability in terms of host species, space (four mature forest sites) and season of the year (dry and rainy), and the mechanisms of infection in the understory of the Los Tuxtlas tropical rain forest. Sixty-five percent of the species surveyed in the dry season (N = 49) and 64.9% of those surveyed in the rainy season (N = 57) were damaged by fungi. Leaf area damaged per plant, on average, was <1% (range: 0.25-20.52%). There was considerable variation in the degree of infection among species, but not among sites and seasons. The survey showed that 43% of the leaves were damaged by herbivores and pathogens concurrently, 16% showed damage by insect herbivory alone, and only 1.4% of the sampled leaves showed damage by pathogens alone. Pathogenicity assays experimentally confirmed that the predominant mechanism of fungal establishment was wounding, such as that caused by herbivory (or other similar sources), and only rarely did infection occur through direct contact (without wounds). The results revealed the omnipresence of leaf fungal infection, although with low damage per plant, and the importance of herbivorous insects in the facilitation of fungal infection in tropical understory plants.

A MAP kinase of the vascular wilt fungus Fusarium oxysporum is essential for root penetration and pathogenesis

The soil-borne vascular wilt fungus Fusarium oxysporum infects a wide variety of plant species by directly penetrating roots, invading the cortex and colonizing the vascular tissue. We have identified fmk1, encoding a mitogen-activated protein kinase (MAPK) of F. oxysporum that belongs to the yeast and fungal extracellular signal-regulated kinase (YERK1) subfamily. Targeted mutants of F. oxysporum f. sp. lycopersici carrying an inactivated copy of fmk1 have lost pathogenicity on tomato plants but show normal vegetative growth and conidiation in culture. Colonies of the fmk1 mutants are easily wettable, and hyphae are impaired in breaching the liquid-air interface, suggesting defects in surface hydrophobicity. Fmk1 mutants also show reduced invasive growth on tomato fruit tissue and drastically reduced transcript levels of pl1 encoding the cell wall-degrading enzyme pectate lyase. Conidia of the mutants germinating in the tomato rhizosphere fail to differentiate penetration hyphae, resulting in greatly impaired root attachment. The orthologous MAPK gene Pmk1 from the rice leaf pathogen Magnaporthe grisea complements invasive growth and partially restores surface hydrophobicity, root attachment and pathogenicity in an fmk1 mutant. These results demonstrate that FMK1 controls several key steps in the pathogenesis of F. oxysporum and suggest a fundamentally conserved role for the corresponding MAPK pathway in soil-borne and foliar plant pathogens.

PTK1, a mitogen-activated-protein kinase gene, is required for conidiation, appressorium formation, and pathogenicity of Pyrenophora teres on barley.

Mitogen-activated protein kinases (MAPKs) are a group of protein kinases that execute a wide variety of roles in cellular signal transduction pathways such as osmoregulation, cell wall biosynthesis, growth, and differentiation. A polymerase chain reaction (PCR) with degenerate primers based on conserved regions of known MAPKs was used to clone the MAPK gene PTK1 from the leaf pathogen Pyrenophora teres (anamorph Drechslera teres), the causal agent of net blotch of barley (Hordeum vulgare L.). The predicted amino acid sequence shows high homology with MAPKs from other phytopathogenic fungi. The gene is present in the genome as a single copy. PTK1 is expressed during in vitro growth on complete medium, under conidiation-inducing conditions and during infection of barley leaves, as shown by reverse transcription-PCR studies. In order to assess the role of PTK1 in the life cycle of P. teres, targeted gene disruption was conducted. Mutants carrying an interrupted copy of the gene were deficient in conidiation, did not form appressoria on glass surfaces or on barley leaves, lost their ability to infect barley leaves, and could not colonize host tissues following artificial wounding.

Biosynthesis of scopoletin in Hevea brasiliensis leaves inoculated with Phytophthora palmivora

Inoculation of resistant (R) and susceptible (S) Hevea brasiliensisleaves with Phytophthora palmivorainduced foliar necrosis and biosynthesis of scopoletin (Scp), considered as a Heveaphytoalexin. The degree of resistance of four clones, as classified by the necrotic lesions, was related to the rapidity and intensity of Scp production. The resistant BPM-24, and marked partially resistant clone PB-235, displayed an early secretion of scopoletin that intensified and lasted longer than the weak partially resistant RRIT251, and susceptible clone RRIM600. The lesion size and amount of Scp after infection were positively correlated to the concentration of spores applied to Hevealeaves. In addition, in leaflets inoculated with high spore concentration, Scp reached the highest level earlier than those with low spore concentration. A fungitoxic effect of Scp on mycelium growth was shown in bioassays; the I50 value tested on Phytophthora palmivora was relatively the same as on Phytophthora botryosa, but much lower than those found on other leaf pathogens of rubber tree, Corynespora cassiicolaand Colletotrichum gloeosporioides. The lesion size and level of Scp upon spore inoculation may be appropriate for classifying Heveaclones according to their R/S with respect to P. palmivora

Ascospores are a source of inoculum of Phaeosphaeria nodorum, P. avenaria f. sp. avenaria and Mycosphaerella graminicola in Western Australia

Ascospores of the wheat pathogens Phaeosphaeria nodorum (Muller) Hedjaroude and Mycosphaerella graminicola (Fuckel) Schroeter were trapped above infected wheat straw in 1990 in an area isolated from other possible sources. Release of ascospores by P. nodorum and M. graminicola, and by the oat pathogen P. avenaria f. sp. avenaria (Weber) Eriksson was monitored in the field between April and September 1991 to determine the availability of ascospores as a source of primary inoculum. The major period of ascospore release of P. nodorum and P. avenaria f. sp. avenaria was from mid-June to late August, which corresponded with the seedling to early flag emergence stage of crop development. The number of ascospores trapped was high, exceeding 4000 ascospores/m3 air/day for P. nodorum and 1000 ascospores/m3 air/day for P. avenaria f. sp. avenaria. The major release of ascospores of M. graminicola was from mid-May to late July, corresponding with the early seedling stage of crop development. The numbers trapped, however, were low and did not exceed 100 ascospores/m3 air/day. P. nodorum was recognised as the predominant wheat leaf pathogen at the trapping site and this was reflected in larger numbers of ascospores of P. nodorum compared with those of M. graminicola.

Patterns and correlates of interspecific variation in foliar insect herbivory and pathogen attack in Brazilian cerrado

Patterns of insect herbivore and leaf pathogen attack are described for 25 plant species (10 trees, 10 shrubs and five herbs) at a Brazilian savanna (cerrado) site. Plant and leaf traits were correlated with interspecific variation in attack by herbivores and pathogens in order to account for differences among plant species. Across all species, pathogen damage was 1.5 times higher than insect damage (17.3% vs. 6.8%, respectively). Most insect damage occurred to young leaves while they were expanding (end of the dry season). In contrast, pathogen attack was low on young expanding leaves at the end of the dry season, increased as those leaves matured in the wet season, but continued to increase through the next dry season. Protein-binding capacity was negatively associated with interspecific differences in insect damage to mature leaves. Protein availability and plant height were positive predictors of pathogen attack among plant species, while leaf expansion rate was a significant negative predictor. Interspecific differences in leaf phenology had little effect on the amount of damage caused by either insects or pathogens. However, new leaves produced during the wet season suffered less insect damage than leaves produced during the dry season, the time of greatest leaf production. Timing of young leaf production affected pathogen attack but the season of escape depended on plant species. In contrast, there was no evidence for escape in space as common species were less likely to suffer high pathogen attack than rare species. New and mature leaf toughness, and time for a leaf to reach full expansion all increased from herbs to shrub to trees, while mature leaf nitrogen decreased in that order.

Early defence responses induced by AVR9 and mutant analogues in tobacco cell suspensions expressing the Cf-9 resistance gene

The interaction between the fungal leaf pathogen Cladosporium fulvum and its only host, tomato, fits the gene-for-gene model. In tomato, the Cf-9 resistance gene product mediates specific recognition of the fungal avirulence gene product AVR9, resulting in a hypersensitive response and resistance. Cf9 tomato leaves respond with necrosis after injection with AVR9, whereas Cf9 tomato cell suspensions do not show defence responses after treatment with AVR9. Here we report on early defence responses induced in Cf-9 transgenic tobacco leaves and Cf-9 transgenic tobacco cell suspensions after treatment with synthesized AVR9 and mutant analogues R08K, F10A and F21A. The necrosis-inducing activity of the AVR9 peptides increased in the order F21A, F10A, AVR9, R08K. An oxidative burst was induced at a much lower AVR9 peptide concentration as compared to medium alkalization and necrosis. Interestingly, the mutant peptide F21A failed to induce necrosis and medium alkalization but did induce an oxidative burst. In all assays, the relative differential activities of the AVR9 peptides were similar to those observed in Cf9 tomato leaves. Both AVR9 and F21A activated a MAP kinase in Cf-9 transgenic tobacco cell suspensions. AVR9 also induced specific cell death in these suspensions. The relation between the induction of early defence responses and necrosis is discussed.

Fungi occurring on Proteaceae in Australia: selected foliicolous species

This paper represents the first of a series dealing with fungi associated with Proteaceae cultivated in Australia. Several leaf pathogens are newly recorded, including Botryosphaeria proteae, Coleroa senniana, Kabatiella proteae, Mycosphaerella jonkershoekensis and Mycosphaerella lateralis from Protea spp. The host range of Phyllosticta telopea is expanded to also include Grevillea, Leucadendron and Leucospermum, and a Pestalotiopsis leaf spot known previously from Africa only is also reported on Protea and Telopea. Cercostigminaprotearum var. hakeae, known from Hakea in South Africa, is also newly reported on Grevillea in Australia. New species described include Leptosphaeria telopeae on Telopea, Mycosphaerella buckinghamiae on Buckinghamia and Ramularia proteae on Protea.

A mitogen-activated protein kinase of the corn leaf pathogen Cochliobolus heterostrophus is involved in conidiation, appressorium formation, and pathogenicity: diverse roles for mitogen-activated protein kinase homologs in foliar pathogens.

Fungal pathogens perceive and respond to molecules from the plant, triggering pathogenic development. Transduction of these signals may use heterotrimeric G proteins, and it is thought that protein phosphorylation cascades are also important. We have isolated a mitogen-activated protein kinase homolog from the corn pathogen Cochliobolus heterostrophus to test its role as a component of the transduction pathways. The new gene, CHK1, has a deduced amino acid sequence 90% identical to Pmk1 of the rice blast fungus Magnaporthe grisea and 59% identical to Fus3 of Saccharomyces cerevisiae. A series of chk1 deletion mutants has poorly developed aerial hyphae, autolysis, and no conidia. No pseudothecia are formed when a cross between two Deltachk1 mutants is attempted. The ability of Deltachk1 mutants to infect corn plants is reduced severely. The growth pattern of hyphae on a glass surface is strikingly altered from that of the wild type, forming coils or loops, but no appressoria. This set of phenotypes overlaps only partially with that of pmk1 mutants, the homologous gene of the rice blast fungus. In particular, sexual and asexual sporulation both require Chk1 function in Cochliobolus heterostrophus, in contrast to Pmk1, but perhaps more similar to yeast, where Fus3 transmits the mating signal. Chk1 is required for efficient colonization of leaf tissue, which can be compared with filamentous invasive growth of yeast, modulated through another closely related mitogen-activated protein kinase, Kss1. Ubiquitous signaling elements thus are used in diverse ways in different plant pathogens, perhaps the result of coevolution of the transducers and their targets.

Mycorrhiza-induced changes in disease severity and PR protein expression in tobacco leaves

The development of leaf disease symptoms and the accumulation of pathogenesis-related (PR) proteins were monitored in leaves of tobacco (Nicotiana tabacum cv. Xanthinc) plants colonized by the arbuscular mycorrhizal fungus Glomus intraradices. Leaves of mycorrhizal plants infected with the leaf pathogens Botrytis cinerea or tobacco mosaic virus showed a higher incidence and severity of necrotic lesions than those of nonmycorrhizal controls. Similar plant responses were obtained at both low (0.1 mM) and high (1.0 mM) nutritional P levels and with mutant plants (NahG) that are unable to accumulate salicylic acid. Application of PR-protein activators induced PR-1 and PR-3 expression in leaves of both nonmycorrhizal and mycorrhizal plants; however, accumulation and mRNA steady-site levels of these proteins were lower, and their appearance delayed, in leaves of the mycorrhizal plants. Application of 0.3 mM phosphate to the plants did not mimic the delay in PR expression observed in the mycorrhizal tobacco. Together, these data strongly support the existence of regulatory processes, initiated in the roots of mycorrhizal plants, that modify disease-symptom development and gene expression in their leaves.

Identification of a locus in arabidopsis controlling both the expression of rhizobacteria-mediated induced systemic resistance (ISR) and basal resistance against Pseudomonas syringae pv. tomato.

Selected nonpathogenic rhizobacteria with biological disease control activity are able to elicit an induced systemic resistance (ISR) response that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Ten ecotypes of Arabidopsis thaliana were screened for their potential to express rhizobacteria-mediated ISR and pathogen-induced SAR against the leaf pathogen Pseudomonas syringae pv. tomato DC3000 (Pst). All ecotypes expressed SAR. However, of the 10 ecotypes tested, ecotypes RLD and Wassilewskija (Ws) did not develop ISR after treatment of the roots with nonpathogenic Pseudomonas fluorescens WCS417r bacteria. This nonresponsive phenotype was associated with relatively high susceptibility to Pst infection. The F1 progeny of crosses between the non-responsive ecotypes RLD and Ws on the one hand, and the responsive ecotypes Columbia (Col) and Landsberg erecta (Ler) on the other hand, were fully capable of expressing ISR and exhibited a relatively high level of basal resistance, similar to that of their WCS417r-responsive parent. This indicates that the potential to express ISR and the relatively high level of basal resistance against Pst are both inherited as dominant traits. Analysis of the F2 and F3 progeny of a Col x RLD cross revealed that inducibility of ISR and relatively high basal resistance against Pst cosegregate in a 3:1 fashion, suggesting that both resistance mechanisms are monogenically determined and genetically linked. Neither the responsiveness to WCS417r nor the relatively high level of basal resistance against Pst were complemented in the F1 progeny of crosses between RLD and Ws, indicating that RLD and Ws are both affected in the same locus, necessary for the expression of ISR and basal resistance against Pst. The corresponding locus, designated ISR1, was mapped between markers B4 and GL1 on chromosome 3. The observed association between ISR and basal resistance against Pst suggests that rhizobacteria-mediated ISR against Pst in Arabidopsis requires the presence of a single dominant gene that functions in the basal resistance response against Pst infection.

In Vivo and in Vitro Observations of Cercospora Mikaniacloa from Hong Kong: Morphology, Microcycle Conidiation, and Potential Biocontrol of Mikania Weed

Cercospora mikaniacola, a leaf pathogen of Mikania micrantha, is reported as a new record for Hong Kong. Morphology of the fungus on the host plant as well as in culture is described and illustrated. Microcycle conidiation is reported and its significance in potential development of mycoherbicide is discussed.

A Taxonomic Reassessment of Phyllachora proteae, a Leaf Pathogen of Proteaceae

Phyllachora proteae is a well known leaf pathogen of Protea spp. In the present study this fungus was recollected from several genera and species of Proteaceae in the Western Cape province of South Africa, and its taxonomy was reassessed. Single ascospore cultures produced a Fusicoccum anamorph in culture, described here as F. proteae. A microconidial synanamorph with narrowly ellipsoidal, brown, thick-walled conidia was commonly associated with F. proteae in culture. Based on its bitunicate asci, as well as pseudothecial and ascospore morphology, a new combination for P. proteae is proposed in Botryosphaeria, as B. proteae.

A taxonomic reassessment of Phyllachora proteae, a leaf pathogen of Proteaceae

Phyllachora proteae is a well known leaf pathogen of Protea spp. In the present study this fungus was recollected from several genera and species of Proteaceae in the Western Cape province of South Africa, and its taxonomy was reassessed. Single ascospore cultures produced a Fusicoccum anamorph in culture, described here as F. proteae. A microconidial synanamorph with narrowly ellipsoidal, brown, thick-walled conidia was commonly associated with F. proteae in culture. Based on its bitunicate asci, as well as pseudothecial and ascospore morphology, a new combination for P. proteae is proposed in Botryosphaeria, as B. proteae.

Rhizobacteria-mediated induced systemic resistance (ISR) in Arabidopsis is not associated with a direct effect on expression of known defense-related genes but stimulates the expression of the jasmonate-inducible gene Atvsp upon challenge.

Selected strains of nonpathogenic rhizobacteria from the genus Pseudomonas are capable of eliciting broad-spectrum induced systemic resistance (ISR) in plants that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). In Arabidopsis, the ISR pathway functions independently of salicylic acid (SA) but requires responsiveness to jasmonate and ethylene. Here, we demonstrate that known defense-related genes, i.e. the SA-responsive genes PR-1, PR-2, and PR-5, the ethylene-inducible gene Hel, the ethylene- and jasmonate-responsive genes ChiB and Pdf1.2, and the jasmonate-inducible genes Atvsp, Lox1, Lox2, Pall, and Pin2, are neither induced locally in the roots nor systemically in the leaves upon induction of ISR by Pseudomonas fluorescens WCS417r. In contrast, plants infected with the virulent leaf pathogen Pseudomonas syringae pv. tomato (Pst) or expressing SAR induced by preinfecting lower leaves with the avirulent pathogen Pst(avrRpt2) exhibit elevated expression levels of most of the defense-related genes studied. Upon challenge inoculation with Pst, PR gene transcripts accumulated to a higher level in SAR-expressing plants than in control-treated and ISR-expressing plants, indicating that SAR involves potentiation of SA-responsive PR gene expression. In contrast, pathogen challenge of ISR-expressing plants led to an enhanced level of Atvsp transcript accumulation. The otherjasmonate-responsive defense-related genes studied were not potentiated during ISR, indicating that ISR is associated with the potentiation of specific jasmonate-responsive genes.

A novel signaling pathway controlling induced systemic resistance in Arabidopsis.

Plants have the ability to acquire an enhanced level of resistance to pathogen attack after being exposed to specific biotic stimuli. In Arabidopsis, nonpathogenic, root-colonizing Pseudomonas fluorescens bacteria trigger an induced systemic resistance (ISR) response against infection by the bacterial leaf pathogen P. syringae pv tomato. In contrast to classic, pathogen-induced systemic acquired resistance (SAR), this rhizobacteria-mediated ISR response is independent of salicylic acid accumulation and pathogenesis-related gene activation. Using the jasmonate response mutant jar1, the ethylene response mutant etr1, and the SAR regulatory mutant npr1, we demonstrate that signal transduction leading to P. fluorescens WCS417r-mediated ISR requires responsiveness to jasmonate and ethylene and is dependent on NPR1. Similar to P. fluorescens WCS417r, methyl jasmonate and the ethylene precursor 1-aminocyclopropane-1-carboxylate were effective in inducing resistance against P. s. tomato in salicylic acid-nonaccumulating NahG plants. Moreover, methyl jasmonate-induced protection was blocked in jar1, etr1, and npr1 plants, whereas 1-aminocyclopropane-1-carboxylate-induced protection was affected in etr1 and npr1 plants but not in jar1 plants. Hence, we postulate that rhizobacteria-mediated ISR follows a novel signaling pathway in which components from the jasmonate and ethylene response are engaged successively to trigger a defense reaction that, like SAR, is regulated by NPR1. We provide evidence that the processes downstream of NPR1 in the ISR pathway are divergent from those in the SAR pathway, indicating that NPR1 differentially regulates defense responses, depending on the signals that are elicited during induction of resistance.

Aerial Plant Surface Microbiology

This book is issued from the 6th International Symposium on the Microbiology of Aerial Plant Surfaces. It is arranged into five principal sections covering the physical and chemical environment of aerial plant surfaces (two chapters); interactions between plant surface micro-organisms and their hosts (four chapters); interactions among micro-organisms in communities on aerial plant surfaces (three chapters); plant surface micro-organisms, agricultural practices and food quality (five chapters); and quantification and modelling of population dynamics of micro-organisms associated with aerial plant surfaces (five chapters). Much of the content is relevant to plant pathologists, particularly those interested in the ecology of leaf pathogens.

Characterization of Colletotrichum Species Responsible for Anthracnose Diseases of Various Fruits.

ilamentous fungi of the genus Colletotrichum and its teleomorph Glomerella are considered major plant pathogens worldwide. They cause significant economic damage to crops in tropical, subtropical, and temperate regions. Cereals, legumes, ornamentals, vegetables, and fruit trees may be seriously affected by the pathogen (3). Although many cultivated fruit crops are infected by Colletotrichum species, the most significant economic losses are incurred when the fruiting stage is attacked. Colletotrichum species cause typical disease symptoms known as anthracnose, characterized by sunken necrotic tissue where orange conidial masses are produced. Anthracnose diseases appear in both developing and mature plant tissues (4). Two distinct types of diseases occur: those affecting developing fruit in the field (preharvest) and those damaging mature fruit during storage (postharvest). The ability to cause latent or quiescent infections has grouped Colletotrichum among the most important postharvest pathogens. Species of the pathogen appear predominantly on aboveground plant tissues; however, belowground organs, such as roots and tubers, may also be affected. In this article, we deal in particular with methods used to identify and characterize Colletotrichum species and genotypes from almond, avocado, and strawberry, as examples, using traditional and molecular tools. The three pathosystems chosen represent different disease patterns of fruitassociated Colletotrichum. Multiple Species on a Single Host Numerous cases have been reported in which several Colletotrichum species or biotypes are associated with a single host. For example, avocado and mango anthracnose, caused by both C. acutatum and C. gloeosporioides, affect fruit predominantly as postharvest diseases (25,40,41). Strawberry may be infected by three Colletotrichum species, C. fragariae, C. acutatum, and C. gloeosporioides, causing anthracnose of fruit and other plant parts (31). Almond and other deciduous fruits may be infected by C. acutatum or C. gloeosporioides (Table 1) (1,5,46,50). Citrus can be affected by four different Colletotrichum diseases (61): postbloom fruit drop and key lime anthracnose, both caused by C. acutatum, and shoot dieback and leaf spot, and postharvest fruit decay, both caused by C. gloeosporioides. Additional examples of hosts affected by multiple Colletotrichum species include coffee, cucurbits, pepper, and tomato. Single Species on Multiple Hosts It is common to find that a single botanical species of Colletotrichum infects multiple hosts. For example, C. gloeosporioides (Penz.) Penz. & Sacc. in Penz. (teleomorph: Glomerella cingulata (Stoneman) Spauld. & H. Schrenk), which is considered a cumulative species and forms the sexual stage in some instances, is found on a wide variety of fruits, including almond, avocado, apple, and strawberry (Table 2) (6,15,31,46). Likewise, C. acutatum J.H. Simmonds has been reported to infect a large number of fruit crops, including avocado, strawberry, almond, apple, and peach (1,5,16,25,27). Examples of other species with multiple host ranges include C. coccodes, C. capsici, and C. dematium (14,56).

Comparison of canola, Indian mustard and Linola in two contrasting environments. II. Break-crop and nitrogen effects on subsequent wheat crops

The main canola-growing region in Australia is southern New South Wales where previous studies showed higher yield and grain protein of wheat growing after brassicas compared with wheat grown after wheat. This advantage, called the break-crop effect, was studied using winter oilseeds in two field experiments, one in this region and the other in central western New South Wales which is generally drier during the growing season, warmer throughout the year and is currently considered marginal for oilseeds. The effect of nitrogen (N) fertilizer on the response of wheat to previous crops was also investigated by considering both the soil N remaining from fertilizer applied to the break crops, and N applied to the subsequent wheat crops. The experiments were conducted over three years, with two phases of an oilseed-wheat sequence and a wheat-wheat control sequence at each site. Both sites had low baseline levels of soil mineral-N and average levels of root-disease inoculum. At the drier site the inoculum of wheat leaf and root pathogens remained during both phases but there were no break-crop effects. The effect of previous crops on the yield and protein of a subsequent wheat crop could be explained by the amount of residual soil mineral N. At the wetter site, wheat responses to previous crops could be explained by the amounts of residual soil mineral N in one phase of the experiment when there was no root disease of wheat. Under these conditions, yield and grain protein generally increased in response to increasing levels of soil mineral N. The exception was the yield of wheat after Linola which decreased when an excessive amount of residual N resulted in greater vegetative biomass, rapid depletion of soil water and decreased yield. In the other phase, when root disease was present, break crops increased yield of a subsequent wheat crop by 30% and grain protein by 1.3% compared to wheat growing after wheat, and among the oilseeds the brassicas gave a greater break-crop benefit than Linola. Application of fertilizer N to wheat growing after wheat failed to compensate for the disadvantage, indicating that residual N was not responsible for the differences. The break-crop benefit of the oilseeds extended to the second successive wheat crop for the phase in which root diseases were present, with increases of 13% in grain yield. The break-crop effect at the wetter site confirms previous observations of this benefit of oilseeds in general, and brassicas in particular, in southern New South Wales. The absence of any break-crop effect at the drier site suggests that root disease of wheat was less severe, possibly because the inoculum was less infective during the dry springs.

Mycoflora and pathogenicity of fungi present on stored seeds from provenances of Eucalyptus pellita

Fungi associated with stored seeds of Eucalyptus pellita were examined and differences in seed germination, seedling mortality and fungal infection for different provenances were determined. A total of 40 fungal species representing 17 genera of Deuteromycotina, four of Ascomycotina and three of Zygomycotina was identified. Of the 40, 26 were seedborne fungi. Several leaf pathogens were found. There were no significant differences in the level of fungal infestation between provenances, but significant differences in seed germination, seedling mortality and fungal infection occurred when seed lots were inoculated with fungi. Seed lot 17854 from Indonesia and 18197 from Papua New Guinea were less affected with regard to seed germination and seedling mortality than the Australian seed lots 18749 and 18775. The inoculation test showed that Aspergillus niger, Harknessia fumaginea and Pestalotiopsis disseminata were able to reduce germination and cause seedling mortality of some seed lots of E. pellita. Drechslera australiensis caused maximum mean seedling mortality (23.65%) but had no effect on seed germination for any seed lot and Syncephalastrum racemosum affected neither seed germination nor seedling mortality.